1. Field of the Invention
The present invention relates to a toner for forming an electrostatic image, which is used in an electrophotographic image forming apparatus, such as a photocopier, a printer, and FAX, and to a developer using the toner, a process cartridge, and an image forming apparatus in which the process cartridge is mounted.
2. Description of the Related Art
A technology for fixing a toner with low energy is desired because of the recent prosperity of environmentally friendly products. There are various ways for achieving such fixing, but among them, there is a strong demand for a toner for forming an electrostatic image, which can be fixed at low temperature.
As a method for lowering fixing temperature of a toner, typically performed is to lower glass transition temperature (Tg) of a toner binder. As Tg is merely made low, however, aggregation (blocking) of powder tends to occur. If the toner powder is aggregated inside an image forming apparatus, operation of a developing device is affected, and there is a case where the developing device cannot be operated. Even if the developing device can be still operated, a toner cannot be supplied, as the toner is aggregated inside a toner container, which leads to low toner density, and formation of defective images.
As a toner is designed to have low Tg, moreover, the toner tends to deposit on a carrier, a photoconductor, and various blades, and therefore defective images may be formed. Accordingly, it is necessary to prevent occurrences of blocking or filming, and to improve anti-blocking property of a toner. Moreover, shelf stability of a toner present at a surface of a fixed image is degraded, as Tg is low. If the fixed image is easily melted and dislocated, the toner may be deposited to another recording medium stacked on the recording medium bearing the fixed image, and therefore it may not be able to store the fixed image for a long period.
Tg is an important factor in the design of a toner binder. In according to a method merely reducing Tg, a toner, which can be fixed by a fixing device temperature of which is set lower than temperature used in conventional art, has not been obtained.
Meanwhile, as a method for achieving anti-blocking property, anti-filming property, and low temperature fixing ability of a toner, use of a crystalline resin as a toner binder has been known for a long time. However, such toner has a problem that hot offset is caused due to lack of elasticity when the toner is melted.
Moreover, as a method for achieving anti-blocking property, anti-filming property, and low temperature fixing ability of a toner, proposed is a core-shell type toner having a shell, formed by a melt suspension method, or emulsification aggregation method (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2009-053695, and 2011-150229). However, these toners are still insufficient to achieve excellent anti-blocking properties and anti-filming properties, while maintaining low temperature fixing abilities.
Furthermore, to solve the aforementioned problem, proposed is a method focusing on a crystalline resin (see JP-A No. 2011-123483). However, such crystalline resin is easily influenced by external conditions (heat history during production, storage, and fixing, and partial phase mixing), and a crystalline structure thereof is not stable, which may adversely affect various properties of a toner, anti-blocking property, and image stability.
In accordance with these conventional techniques, moreover, the crystalline polyester resin sharply melts compared to a non-crystalline polyester resin, and therefore these toners can achieve low temperature fixing ability. It is possible to achieve both low temperature fixing ability and heat resistant storage stability of a toner according to these conventional techniques, however, there are problems, particularly in the case where a toner is used in a high-speed device, that the toner forms aggregates as stress applied to the toner in the developing device is large, and a white missing area (transfer missing) is formed on an output toner image due to a doctor blockage. In case of a toner containing a crystalline polyester resin, moreover, there is a problem that the toner forms aggregates in high temperature high humidity environment.
Accordingly, it is current situation that there is needs for a toner, which has low temperature fixing ability, anti-blocking property, and anti-filming property, and capable of preventing transfer missing.